There are three primary goals in this proposal. The first is to understand the molecular details of how the CO sensing transcription factor CooA functions. The binding of CO to the heme iron results in very large structural changes some 20 from the CO. These structural changes switch CooA to the on-state such that it specifically recognizes 5' DNA regulatory sequences where CooA promotes specific gene transcription involved in CO oxidation. CooA is a paradigm for how small gaseous ligands lead to large allosteric transitions coupled to important signal transduction processes. In addition, CooA exhibits exquisite ligand selectivity such that only CO results in the allosteric transition and no other biological diatomic gases such as NO and O2. Given that gas sensing now is recognized as a critically important biological process throughout the biosphere, uncovering the structural details of ligand selectivity in CooA is especially important. A second goal is to understand the structural details of heme transport in pathogenic bacteria. Such pathogens must acquire nutrients from the host and one such nutrient is iron. Pathogenic bacteria have developed a complex system where host heme is transported into the cytosol where it is degraded thus releasing iron. The goal of this aim is to determine the structures of all protein components including specific protein complexes. Given that these proteins are unique to bacterial pathogens, they can potentially provide useful therapeutic targets. The third goal is to study structure-function relationships in peroxidases. Recent advances in coupling x-ray data collection, single crystal spectroscopy, and novel composite data collection methods have made it possible to determine the crystal structure of unstable enzyme intermediates. This goal now is to focus on these intermediates in redox protein-protein complexes. A new crystal structure of a peroxidase-cytochrome c complex has been solved which has provided a new system for probing inter-protein electron transfer.